Train No.1206 and K27 go from Tianjin to Dandong. Train No.1206 departs at 04:35 and reaches at 18:20, while Train No.K27 departs at 19:06 and arrives at 07:17. In my opinion,you can take the Train No.K27. There are two reasons: first, you can enjoy your supper and then go to take it. It's easy to go to the railway station before 19:06. But if you take the Train No.1206, you should reach the railway station before 04:35. So early and the transportation would be not so convenient. Second, K27 is a fast-speed train. In China, the train numbers with an alphabet 'K' means 'faster'. So, it can help you reach Dandong a little earlier. Of course, the ticket of the Train No.K27 is dearer than the Train No.1206. Train No.K27 is an overnight train. I suggest you purchase an sleeper ticket (hard sleeper or soft sleeper).